selftests: bpf: modify urandom_read and link it non-statically

After some experiences I found that urandom_read does not need to be
linked statically. When the 'read' syscall call is moved to separate
non-inlined function then bpf_get_stackid() is able to find
the executable in stack trace and extract its build_id from it.

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/urandom_read.c

@@ -7,11 +7,19 @@
 
 #define BUF_SIZE 256
 
+static __attribute__((noinline))
+void urandom_read(int fd, int count)
+{
+       char buf[BUF_SIZE];
+       int i;
+
+       for (i = 0; i < count; ++i)
+               read(fd, buf, BUF_SIZE);
+}
+
 int main(int argc, char *argv[])
 {
 	int fd = open("/dev/urandom", O_RDONLY);
-	int i;
-	char buf[BUF_SIZE];
 	int count = 4;
 
 	if (fd < 0)
@@ -20,8 +28,7 @@ int main(int argc, char *argv[])
 	if (argc == 2)
 		count = atoi(argv[1]);
 
-	for (i = 0; i < count; ++i)
-		read(fd, buf, BUF_SIZE);
+	urandom_read(fd, count);
 
 	close(fd);
 	return 0;